1. Field of the Invention
This invention relates generally to apparatus for generating a digital audio signal and, more particularly, is directed to a digital audio signal generating apparatus suitable in the application to electronic musical instruments, a sound effect generator for amusement machines and the like.
2. Description of the Prior Art
As a sound source for electronic musical instruments or a sound effect sound source for amusement machines, the following one is proposed in the prior art. According to this previously-proposed sound source, a rectangular wave signal, for example, is supplied to a plurality of preset frequency dividers each having different frequency-dividing ratio and different duty ratio. Sound source signals (i.e. so-called voices) from the respective frequency dividers are synthesized in a proper level. In that case, the original oscillation waveform may be a triangular wave, a sinusoidal wave or the like.
In some musical instruments such as piano and drums, the total sound generating period is divided to provide 4 intervals such as attack period, decay period, sustain period and release period, and the amplitude (level) of the signal in each interval presents a peculiar changed condition. Accordingly, a so-called ADSR (attack, decay, sustain, release) control is performed so as to cause the signal level of each voice to be changed similarly.
On the other hand, as a sound source for musical instruments, a so-called FM sound source is known, in which a sine wave signal is frequency-modulated (FM) by a sine wave signal having a low frequency. According to this FM sound source, a modulation factor is made as a function of time and various kinds of sound signals (sound signal means an audio signal in this specification) can be obtained by the lesser sound source. The sound effect sound source may be a noise component (i.e. white noise component and the like).
In order to obtain real sounds of various musical instruments by using the afore-mentioned so-called electronic sound source, very complex signal processing is required and hence, the circuit scale of the audio signal generating apparatus is made large.
To solve the above-mentioned problem, recently, a so-called sampler sound source is proposed, in which real sounds of various musical instruments are digitally recorded, written in a memory (ROM) and a signal of a predetermined musical instrument is read from this memory.
In this sampler sound source, in order to save the storage capacity of the memory, the digital audio signal is data-compressed and written in the memory, while the compressed digital signal read from the memory is data-expanded and is re-converted to the original digital sound signal. In this case, only a signal of sound having particular is written in the memory for each musical instrument, and the signal read from the memory is pitch-converted to generate a fundamental frequency signal of sound having a desired pitch and loudness.
Further, a signal waveform, appearing in the initial stage of sound generation and peculiar to each musical instrument, is directly written in the memory and is read out of the memory. This signal waveform is what might be called a formant, and the formant means, in the case of, for example, piano, a sound such as an operation sound and the like generated when the pianist touches a keyboard of the piano to cause a hammer to strike a key. In that case, a repetitive waveform portion of a fundamental cycle is written in the memory for only one cycle and is repeatedly read from the memory.
More specifically, upon reproduction, as shown in FIG. 1, subsequent to a formant component a of short time period, a fundamental frequency signal component b, which is formed of repetitive waveforms p, is obtained and thus a sound of a desired musical instrument can be obtained. In that event, in the case the sound of a piano or the like, the natural sound of the musical instrument can be reproduced by gradually decreasing the level of the waveform p in accordance with a predetermined rule.
According to the above-mentioned musical instruments if a reverberation sound or the like is added to the reproduced musical instrument sound, then it will be possible to reproduce sounds of various tones. In particular, when a digital audio signal generating apparatus is employed as the sound effect generating apparatus for amusement machines, it is frequently requested to add the reverberation sound to the reproduced sound in order to gain presence satisfactorily.
When the reverberation sound is added to the digital audio signal, however, the digital audio signal is temporarily stored in a memory such as a random access memory (RAM) or the like and is delay-processed by this memory, thereby generating a reverberation sound. This requires a memory of large storage capacity in which the digital audio signal is stored, which fact makes the apparatus complicated in arrangement.
Further, according to the previously-proposed reverberating apparatus, when control data from a central processing unit (CPU) is not defined because the power switch of the apparatus is just turned ON, a delay processing area is erroneously set in the sound source data writing area of the memory, which causes the apparatus to malfunction.
Further, when the FM operation is performed in order to gain various sounds in the sampler sound source, a signal source for frequency-modulation is required, which causes the circuit arrangement to become complicated.
Furthermore, when an amplitude-modulation (AM) is performed in order to achieve performance effect, there is then presented a similar problem.
In addition, the memory which temporarily stores the sound source data and the control program required for processing the sound source data must have a relatively large storage capacity, which unavoidably causes the circuit arrangement to become complicated.